It is customary to filter the air provided to occupied spaces by heating, ventilating, and air conditioning (HVAC) equipment. A typical HVAC system for a residence for example, has a fan that while operating draws air present within the occupied space through a return air intake opening and into a plenum or air duct leading to a furnace or air conditioner (generically, air processor) for reprocessing.
HVAC systems frequently cause the reprocessed air to pass through a filter to remove particulate contamination. One convenient and effective way to do this is to filter the air as it enters the return air intake opening in the plenum or duct leading to the air processor. This prevents dirty air from reaching heat exchanger surfaces.
The filter may be a simple mechanical filter with a disposable or renewable element, or may be electronic. The following description pertains to mechanical filters that collect these particles on or within the filter material through which the filtered air passes.
It is helpful at this point to define terms applying to mechanical air filters that will be frequently used in the description to follow. The medium of an air filter is the actual material that performs the filtering function. The air filter element is the entire disposable unit including the medium and any support structure that is installed in a filter housing and is discarded after the medium material has become clogged with air contaminants.
In residential systems, the medium is usually formed in a nominally one inch (2.5 cm.) thick rectangular shape. The filter element usually includes a frame forming the periphery of the filter element as a support structure. The medium is typically either a woven glass fiber mat, or pleated paper or other sheet-type medium material. The breadth and width dimensions of these filter elements vary to conform to the dimensions of the opening in which the element is to be installed, but typically are each on the order of two feet (61 cm.).
In a common design, the frame comprises flexible cardboard edging having a U-shaped cross section enclosing the medium's edges and a small portion of the medium's periphery adjacent the edges. The frame provides stiffness for the filter element and seals the edges against most air leakage around the filter element. This filter format will be referred to hereafter as a shallow filter element or shallow format filter.
The return air intake openings in which these filters are installed typically have annular or inwardly projecting sheet metal or plastic flanges around the entire interior periphery of the opening. The flanges' outer surfaces all lie in a common plane and are set back from the opening a short distance.
The filter's frame is pressed against the flange's outer surface by force from a grille cover having an internal ridge bearing against the filter frame's outer surface to thereby create a tight seal between the outer intake flange surfaces and the inwardly facing filter frame surface. This tight seal forces most all of the air entering the plenum to pass through the filter element medium.
As one would expect, different types of air filters have different efficiencies. “Efficiency” in this context refers to the percentage of the total number of particles in the air stream within a given size range entering the filter that the filter element can catch. The efficiency of filters varies with different particle size ranges. For example, a high efficiency filter medium can catch a significant percentage of particles whose size is on the order of 0.3 micron, where a low efficiency medium catches relatively few of them.
There is also the consideration of overall efficiency as opposed to filter medium efficiency. Overall efficiency takes into account the air leakage around a filter element mounted in its housing. Leaking air is of course completely unfiltered. Its particle load pollutes the stream of filtered air, resulting in an overall efficiency lower than the medium efficiency.
But efficiency is not the only measure of medium quality. It is also important that a filter not create a large pressure drop in the air passing through it. A large pressure drop requires a more powerful fan to force the required air volume through it. And if the pressure drop is too great, the medium will deflect and perhaps even burst or tear as the load of trapped debris obstructs the air passages through the medium.
The amount of pressure drop presented by a particular medium depends largely on the number of voids or openings per unit area of the medium, on the average minimum cross section area of the pores, and of course on the total area of the medium through which the air flows. To a lesser extent, pressure drop is also dependent on the medium thickness, in the same manner that a long duct creates more resistance to air flow through than does a short duct, other things being equal.
Obviously, as a filter element loads up with debris during use, its pressure drop increases. This leads into a further consideration for filters, that of carrying capacity and filter element life. “Carrying capacity” refers to the number of particles the filter element can catch or hold per unit area projected to the air stream before clogging up to a point where the ability to remove particles is impaired and/or the pressure drop across the filter element becomes unacceptable. (“Dust-holding” capacity is an industry term that we intend to be substantially equivalent to carrying capacity.) Other things being equal, carrying capacity is directly related to total medium area. The capacity of mat filters which trap some of the particles within their volume may also depend to some extent on their thickness. Carrying capacity is one factor in determining the life of the element and thus the cost of filtering the air.
Filter technology advances have led to improvements in each of these characteristics. Nevertheless, it is still true that there are tradeoffs between efficiency, pressure drop, and carrying capacity. For example, as a filter medium becomes more efficient, its pressure drop typically increases because the individual passages through the medium become smaller, other things being equal. Of course, it may be possible to add more passages per unit area, but this is not a trivial problem.
Increasing the filter efficiency will often reduce the carrying capacity of the filter element. Often, higher efficiency produces a higher initial pressure drop. Thus as the filter element loads up with particles, the pressure drop reaches an unacceptable level more quickly.
An easy way to minimize pressure drop and maximize capacity is to increase total medium area. This fact has led to the development of the pleated filters mentioned. These pleated filters are made from a long strip of sheet filter medium which is folded back and forth on itself accordion-fashion to form a series of pleats. So long as the adjacent pleat panels do not touch each other the air can easily flow through the individual pleats.
In order to maintain spacing of adjacent pleats from each other under the force created by the normal pressure drop across the medium, it is possible to insert combs on the downstream side of the medium that have individual teeth between each pair of adjacent pleat panels. The teeth prevent adjacent pleats from collapsing against each other.
Improved filter media have been developed whose pressure drop and carrying capacity is superior to that of shallow format mat and pleated filters. These media typically have relatively deep pleats (4-5 in. or 10-12.5 cm.) to provide a relatively large medium area providing lower pressure drop and better carrying capacity. These deep pleat elements are intended for use in return air ducts having intake openings capable of receiving such filter elements.
In one design the filter elements collapse into a relatively small volume for shipping. They have relatively rigid cardboard or plastic end strips or panels that detachably mate with reusable side panels to form a reasonably rigid rectangular filter element. See U.S. Pat. No. 5,840,094 issued on Nov. 24, 1998 to Osendorf, et al. ('094) for an example of such a collapsible filter medium which can be assembled into a deep format pleated filter element using a pair of special side panels. The filter element assembly is mounted in the return air intake, placing the filter element directly in the return air stream.
Collapsible filter media have the distinct advantage of compactness during shipping. But the time and effort required for assembling collapsible filter media for use is one disadvantage of them. The many pleats each require a tooth of the comb, whose insertion between each pair of pleats is time-consuming. And overall filtering efficiency suffers because of difficulty in providing a total air seal between the intake flanges of the return air duct and the cardboard sides of the filter element.